January-February 2007 GSA Bulletin media highlights

Boulder, CO -- Articles are now posted in the GSA BULLETIN "In Press" area of the journal Web site. Geology topics of interest include: two New Zealand supervolcanoes that erupted only days or weeks apart; remarkably detailed K-T boundary evidence from the tropical Atlantic's Demerara Rise supporting a single impact event; and human impact on continental erosion and sedimentation.

Ossipee ring dyke represents an annular intrusion of solidified magma following the eruption and collapse of a supervolcano 121 million years ago in New Hampshire. The dyke contains evidence of processes occurring in the magma chamber preceding, during, and after the eruption. Physical and geochemical results indicate that mafic magma remelted an accumulation of crystals near the floor of a "mushy" silcic magma chamber. Kennedy and Stix further suggest that this remelted mushy material mingled with the rest of the chamber driven by vortices associated with caldera collapse. The mushy magma solidified in the ring dyke between the chamber and the surface as collapse slowed. Remelting and remixing of crystal mush are important in many supervolcanic eruptions.

Large volcanic eruptions of greater than 100 cubic kilometers of magma are generally rare and random events worldwide. The volcanoes associated with these large eruptions are popularly known as “supervolcanoes” but volcanologists refer to them as caldera volcanoes, volcanoes which leave depressions in the earth up to tens of kilometers in diameter (for example, Yellowstone). In this paper, Gravley et al. document geologic field evidence for two large eruptions, in the Taupo Volcanic Zone in New Zealand, separated in time by only days to weeks but from caldera volcanoes spaced 30 kilometers apart. Such a close pairing of supervolcano eruptions has not been documented previously and provides us with a new understanding of the potential linkage between distinct and geographically separated caldera volcanoes.

Takena Formation of the Lhasa terrane, southern Tibet: The record of a Late Cretaceous retroarc foreland basin

Tibet before being squeezed between India and China: The Tibetan plateau is considered by many to be the largest topographic feature that has existed on the surface of the Earth for the past billion years, yet the timing and manner in which this area was uplifted remain unclear. In this article, Leier et al. provide evidence that before this area became the large plateau of today, it resembled a mountain range similar to the modern-day Andes in South America. Sediments deposited by ancient rivers that traversed the region 100–65 million years ago provide evidence that the southern portion of the Tibetan plateau was once occupied by a series of volcanoes and mountains. The volcanoes, mountains, and contemporaneous river deposits were then deformed and elevated to even higher altitudes roughly 55 million years ago as India, which had been migrating northward toward southern Asia, collided with the southern margin of China. Study of the pre-collisional geography of southern Tibet provides new information on how and why Tibet has become a plateau of unparalleled size and altitude.

A Pliocene ignimbrite flare-up along the Tepic-Zacoalco rift: Evidence for the initial stages of rifting between the Jalisco block (Mexico) and North America

New 40Ar/39Ar dates on rhyolitic ignimbrites in the Tepic-Zacolaco rift of western Mexico reveal a previously unrecognized voluminous flare-up of volcanism between 5 and 3 Ma. The eruption rate during this time period was an order of magnitude higher (hundreds of m/m.y.) than that documented in the Tepic-Zacoalco rift over the last 1 m.y. and has implications for the tectonic history of western Mexico. The Tepic-Zacoalco rift, together with the Colima rift and the Middle America trench, bound the Jalisco block, a portion of western Mexico that may be moving independently of North America. The voluminous ignimbrite flare-up in the Tepic-Zacoalco rift at 5–3 Ma may reflect the initial stages of rifting of the Jalisco block from North America. The authors argue that this volcanic flare-up, coupled with rifting, may be analogous to what occurred prior to the transfer of Baja California from North America to the Pacific plate.

Ironstone bodies of the Barberton greenstone belt, South Africa: Products of a Cenozoic hydrological system, not Archean hydrothermal vents!

Bodies of the iron oxide goethite and hematite associated with 3.4-billion-year-old rocks in the Barberton greenstone belt, South Africa, have been previously interpreted as the Earth’s most ancient submarine hydrothermal vent deposits and have yielded putative evidence about Archean hydrothermal systems, ocean composition and temperature, and early life. This paper summarizes geologic evidence that the iron oxides were deposited on and immediately below the modern ground surface by active groundwater and spring systems, probably during periods of higher rainfall within the last few million years. These deposits represent a remarkable, quite young iron oxide–depositing hydrologic system but provide no information about conditions or life on the early Earth.

Geochemical and age constraints on the formation of the Gorda Escarpment and Mendocino Ridge of the Mendocino transform fault in the NE Pacific

The Mendocino Transform Fault is the seismically active boundary that separates two major tectonic plates in the NE Pacific Ocean (the Gorda plate from the Pacific plate). This important plate boundary is also the northernmost extension of the San Andreas fault system. Most of the active transform zone is a conspicuously steep, north-facing cliff formed from uplifted oceanic rocks thinly covered by ocean sediments. Kela et al. used a remotely operated vehicle to study the rock outcrops and collect samples for geochemical analysis. The compositions of the igneous rocks (tholeiitic basalts and alkaline basalts) provide clues about the history of this critical boundary. The results of this study suggest that these rocks now plastered on the uplifted face of the Mendocino Transform Zone actually formed on the southernmost Gorda Ridge from 23 to 11 Ma during a period of rift failure, abandonment, and alkaline seamount volcanism. The almost intact slabs of ocean crust were then slivered off the Gorda plate and transferred onto the Pacific plate by the northward progression of the Mendocino Triple Junction and Mendocino Transform zone in conjunction with the development of the San Andreas System.

Impact and extinction in remarkably complete Cretaceous-Tertiary boundary sections from Demerara Rise, tropical western North Atlantic

Sections on Demerara Rise (western tropical Atlantic) contain the most complete Cretaceous/Paleogene (K/T) boundary intervals yet described from Deep Sea Drilling Project/Ocean Drilling Program cores. The sections contain the uppermost Maastrichtian P. hantkeninoides and lowermost Danian P0 planktic foraminiferal zones (both firsts for deep-sea cores) in addition to well-resolved anomalies in Ir and other siderophile elements. Foraminiferal turnover at the boundary is dramatic and faunas above and below are separated by an ~2 cm thick, graded spherule bed of impact origin. No evidence of impact is found elsewhere in the sequence. Sedimentological and paleontological complexities are minor, and the sections provide no support for multiple impacts or other stresses leading up to or following the deposition of an ejecta bed. The sections preserve fine details of the sedimentological and paleontological expression of the K/T event on Demerara Rise, including the record of suspension of a portion of the seafloor minutes after the impact, deposition of a primary airfall of spherules over the subsequent hours to weeks, and a well-resolved record of the recovery and radiation of foraminifera over the first two million years of the Paleocene. That is, the K/T record on Demerara Rise is remarkable in the degree to which it follows simple predictions given a mass extinction caused by a single impact.

Effects of sediment pulses on channel morphology in a gravel-bed river

Hillslope erosion in mountainous watersheds is often dominated by rare but large landslides and debris flows, events that often occur after fires. The delivery of these large sediment pulses to rivers and streams occupying valley bottoms can severely affect fluvial and biological processes and understanding how the channels process these pulses is critical to understanding how and the speed at which they recover. In July 2001, intense rainfall triggered numerous debris flows in a severely burnt watershed in the Sapphire Mountains of Montana. Ten large debris flow fans were deposited on the valley floor and investigations focused on the channel response to these inputs of sediment. Hoffman and Gabet found consistent patterns of response at each debris flow deposit. For example, fine-grained sediment was deposited upstream of the fans whereas, downstream of the fan, the channel was coarse-grained and braided. Additionally, the authors chronicle the delivery of large woody debris, an important component of fish habitat, by the debris flows. With the increase in fire frequency predicted with global warming, Hoffman and Gabet anticipate that these types of events will become more common, possibly overloading mountain streams and leading to widespread aggradation.

This study focuses on deciphering lake-level fluctuations at Walker Lake, Nevada, during the past 4,000 years. Over the past one hundred years, the surface of Walker Lake has dropped by about 50 m (150 m) in response to upstream diversions of the Walker River. As lake level declined, the river has incised into the former lake bed, exposing interbedded stream, delta, and lake deposits that reflect past lake-level changes. Documentation of the different sedimentary environments combined with their elevations and radiocarbon dating has led to a new chronology of lake-level fluctuations. During the last 4,000 years, Walker Lake has repeatedly fluctuated between high and low levels in response to climate and the diversion of the Walker River out of and back into the Walker River basin. The results of this study help put the current desiccation of Walker Lake into a longer-term perspective.

While mountain uplift and erosion have been the most important of all geomorphic processes in shaping the surface of the Earth over most of geologic time, their dominance was exceeded roughly one thousand years ago by the rock- and soil-moving activities of humans. Through agricultural activity alone, humans have displaced on the order of 20,000 Gt of soil through cropland erosion, an amount about 1,000 times the annual load (~21 Gt/y) of all global rivers combined. This is enough to cover the state of Rhode Island to a depth of almost 3 kilometers, or the entire Earth landscape to a depth of about 6 centimeters. Despite humankind’s recent and significant impact on continental erosion and sedimentation, since 1961 global cropland area has increased by about 11% while the global population has effectively doubled. The net effect of these changes is that per capita cropland area has decreased by about 44% over the same time interval. This is ~25 times faster than the rate of soil area loss resulting from human-induced erosion. In a context of per capita food production, the considerable effect of humans on continental erosion is largely insignificant when compared to the impact of population growth.

Computer simulations of landscape development are often used to help understand the details of geologic processes at the Earth’s surface. While they are often able to produce synthetic landforms that appear similar to natural landforms, calibrating them using actual rates of surface processes such as river erosion into bedrock and slope failure is difficult. DeLong et al. present a combination of field mapping, topographic analysis, and geochronology from Cuyama Valley, in southern California, that they used to calibrate a computer model that simulates slope and river channel erosion in actively uplifting areas. DeLong et al. identify several characteristics of natural landscapes that are quantifiable and allow for successful comparison of computer-generated landscapes to actual landscapes. Using their results, the authors calibrate a common equation that predicts river erosion into bedrock, the “stream power law.”

During subduction, the process in which one plate underthrusts the other one sinking in the fluid mantle, earthquakes occur along the plunging surface representing the boundary between lower and upper plate, an area referred to as the décollement zone. Surprisingly, not all the extent of the décollement is seismic, since the earthquakes are concentrated at a depth interval ranging from roughly 4 km to 10–15 km (seismogenic zone). One of the biggest goals of the scientific community is to understand what controls and determines these up-dip and down-dip limits of seismicity. In this paper, Meneghini and Moore have focused on a fossil example of the décollement zone cropping out along the coast of Rodeo Cove, in the Marin Headlands of San Francisco. The outcrop is a shear zone 200 m thick that the authors have interpreted to be active at seismogenic depth. This paper suggests that hydrofracture of an important volume of overpressured fluid accompanied deformation along that portion of the décollement zone. Various geologists view the cyclic circulation of overpressured fluids into rocks as one of the possible processes promoting seismic slip along a surface. Although debate remains strong, Meneghini and Moore attempt to show a relationship between the observed cycling fluid injection and the seismic cycle.

Four magmatic fabrics in the Tuolumne batholith, central Sierra Nevada, California (USA): Implications for interpreting fabric patterns in plutons and evolution of magma chambers in the upper crust

This paper documents preservation of four distinct magmatic fabrics in a composite plutonic body (the Tuolumne batholith, Sierra Nevada, California) and discusses their general significance for the interpretation of magmatic fabrics patterns in plutons and the evolution of magma chambers in the upper crust.

This article represents a new synthesis of the metamorphic history of the oldest rocks preserved in Grand Canyon National Park. Dumond integrates new petrologic and microstructural data with previously published structural data and geochronology to provide a more complete record of the mountain-building events that led to burial and exhumation of 1.84–1.68-billion-year-old rocks in the Upper Granite Gorge of the Grand Canyon. The study shows that the Upper Granite Gorge preserves an extensive, near-isobaric level of continental crust that came together at 25 km depths nearly 1.7 billion years ago. The region was uplifted to 12 km depths 1.68 billion years ago due to erosion synchronous with crustal thickening. Comparison of pressure-temperature data for a number of rocks along this ~70 km-long transect demonstrates that the large tectonic blocks of the Upper Granite Gorge preserve >100–250°C differences in peak temperature with virtually no variation in pressure. Development of dramatic temperature gradients and discontinuities without breaks in crustal level is attributed to magmatic heat delivered by granite dike complexes and local transcurrent displacements along block-bounding shear zones over a ~15–20 Ma time interval. This study documents a region where temperature differences and crustal heterogeneity exerted a major influence on the strength of continental crust. Cold tectonic blocks prevented pervasive channel flow of hot, partially molten tectonic blocks as the crust was shortened.

The architecture of brittle postorogenic extension: Results from an integrated structural and paleomagnetic study in north Calabria (southern Italy)

The Mediterranean region represents a difficult but intriguing study area for geologists. In this area, over a long period of time the African and European plates have been colliding, forming part of the Alpine-Himalayan chain. Continental extensional tectonics at the rear of contractional orogenic belts is well documented in the Mediterranean region, where Tertiary extensional processes, active at the back of subduction zones, overprinted the early Alpine compressional structures. This process led to extensive attenuation of the orogenic crustal section and to the deposition of thick sedimentary sequences in the newly formed tectonic depressions. In this work, Cifelli et al. use structural analysis and paleomagnetic investigations to study the extensional processes accompanying the subduction process in the Calabrian Arc (southern Italy), an area where subduction is still active. The authors investigate the sedimentary sequences exposed in north Calabria related to extensional tectonics in order to define the brittle postorogenic evolution of the inner sector of the Calabrian Arc and to better define the extensional processes active in the back-arc region of the south-eastward migrating subduction system.

Late Quaternary slip rates along the Sierra Nevada frontal fault zone, California: Slip partitioning across the western margin of the Eastern California Shear Zone–Basin and Range Province

One of the most prominent geomorphic features within the southwestern United States is the Sierra Nevada, a mountain range with a mean elevation of 2800 m above sea level, which is bounded along its east flank by a normal fault zone, the Sierra Nevada frontal fault zone. New data from geologic and geochronologic investigations along the southern part of this fault zone indicate that it has remained tectonically active at a vertical slip rate of 0.2–0.4 ± 0.1 mm/yr throughout the last 125,000 years. These slip rates are similar to slip rates documented along normal faults to the east within the Basin and Range Province. Slip along this part of the eastern flank of the Sierra Nevada is partitioned into three components, subordinate normal, intermediate oblique (normal and right-lateral), and dominant right-lateral slip distributed across three subparallel, closely spaced faults—the Sierra Nevada frontal fault zone, the Lone Pine fault, and the Owens Valley fault, respectively. These observations are consistent with GPS (global positioning system) data that indicate that right-lateral deformation dominates the southwestern margin of the United States.

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5 December 2006
Geological Society of America
Release No. 06-55

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